The invention relates generally to the field of analog signal processing and frequency control and more particularly to surface acoustic wave devices and the resonant frequencies thereof.
In the field of surface acoustic wave (SAW) devices, there is a need for precision frequency in high Q (mechanical response), narrowband SAW devices. Typically these devices operate at hundreds of megahertz (MHz) with bandwidths of only a few hundred kilohertz (kHz). Probable uses require low-loss, high stability frequency selection or frequency control elements which are required to operate directly at UHF/microwave frequencies including preselector filters and frequency control elements for oscillators or synthesizers for receiver front-ends. The devices, prior to any tuning, can be made to operate very close to the desired resonant frequency, however, precision frequency trimming is required to satisfy the very small required frequency tolerances.
Frequency trimming procedures for SAW devices that have been and are now being utilized are reviewed by Thompson in U.S. Pat. No. 3,864,161, issued Feb. 4, 1975, and by R. Fischer and L. Schulzke, in the paper, "Direct Plating to Frequency--A Powerful Fabrication Method for Crystals with Closely Controlled Parameters," at pages 209-213 of the Proceedings of the 30th Annual Symposium on Frequency Control--1976. Some of the methods discussed include plasma etching of the transducer structure, external tuning, dielectric mass-loading and application of metal overlay films. A major disadvantage to plasma etching is that the method results in weakening of the crystal structure increasing the possibility of stress fractures during the life of the device. External trimming has required laborious hand adjustment involving a high degree of inaccuracy and inordinate handling of the unsealed device. All of the techniques mentioned are performed before packaging and sealing the device. As such, with the resonance frequency set before packaging and sealing, there is no accounting for or adjusting of any stray capacitance associated with the packaging procedure. In addition, the above-cited methods require individual device tuning, not conducive to a production environment.
One variation of the dielectric mass-loading technique, the subject of U.S. Pat. No. 4,107,349 entitled "Method of Adjusting the Frequency of Piezoelectric Resonators", which was granted to Vig on Aug. 15, 1978, provides a method for adjusting the frequency, either before or after sealing, utilizing the deposition of a polymer on the resonator. The present invention makes use of an alternative polymer deposition technique and will therefore be distinguished herein. Vig, as stated above, uses deposition of polymers on the surface of the resonator to effect frequency trimming. The change in frequency results from the deposition of the mass itself unlike the present invention wherein is utilized the property of elastic deformation pursuant to curing the polyimide polymer. Vig's pre-sealing method involves placing the unsealed device in an evacuated enclosure which is subsequently filled with a polymerizable gas. The adjustment is effected by polymerizing the gas with radiation from an external source. The amount of polymer deposited or formed on the resonator causes the frequency change. A similar method is utilized (by Vig) after sealing; such that, a polymerizable gas is sealed under a fused quartz window within the resonator. At the time when frequency adjustment is required, the resonator is then subjected to radiation through the quartz window. The latter method requires excess polymerizable gas to be sealed within the element in amounts which could change the frequency of the resonator by large increments. Therefore, after the desired frequency has been attained, the remaining gas is polymerized by directing the radiation for deposition onto an area of the resonator away from the vibrating section. Vig's method, in both instances, absolutely requires that the deposition be on the resonator's active crystal element, and therefore, when applied to SAW devices, the deposition must be in the path of the surface acoustical waves. Since the waves travel on the surface of the element, the deposited polymer will have a damping effect on the waves and thereby degrade the response of the device, decrease the mechanical quality factor, Q, and increase the rate of aging of the element.
The aforementioned patent of Vig deals solely with frequency adjustment by mass-loading which effect invariably produces a frequency lowering. This is acknowledged explicitly of Column 1, lines 54 and 55 of said patent, as well as in claims 24 and 25 thereof. No provisions are made for upward adjustment of frequency in the Vig patent. However, it is well-known in the art that resonant devices of both bulk and surface acoustic wave varieties often age downward in frequency. It is highly desirable, therefore, to have a method of frequency adjustment that is capable of either positive or negative frequency adjustment. This may be done by the method of the instant invention.
It is therefore the object of this invention to provide a means for precisely trimming the resonance frequency of a surface acoustic wave device thereby changing the resonance frequency by a small fraction of the bandwidth.
It is another object to provide a means for fine-tuning the frequency of a surface acoustic wave device in order to adjust the operational resonance to within a few kilohertz (kHz) of that which is desired.
It is another object to provide a means for precisely adjusting the resonance frequency of a surface acoustic wave device in a manner capable of either increasing or decreasing the resonance frequency as required.
It is still another object to provide a technique permitting tuning to be completed after the device is sealed thereby compensating for any stray capacitances associated with the packaging procedures which are otherwise conducted after trimming.
Another object of the present invention is to provide a frequency trimming process amenable to a production environment, allowing alignment of frequency on several devices in one operation.
Finally, it is an object of the present invention to provide a means for trimming the frequency of a surface acoustic wave device without contributing to an increased rate of aging, degrading the response of the device, or decreasing the mechanical quality factor of the elements.